MILLER EFFECT

While designing amplifiers, engineers may assume that the internal capacitances in the transistor are very small com­pared to the external capacitances. But in reality, capaci­tances do exist between the base and emitter (CBE) as well as between base and collector (CBC). This is shown in Fig. 4. It can be mathematically shown that the total input capacitance

CI = CBE + (1 + AV )(CBC)

In other words, the total input capacitance is the parallel combination of CBE and (1 + AV)CBC. The base-collector capac­itance has been amplified by a factor of 1 + AV. This is called the Miller effect.

As mentioned earlier, as the frequency increases, the value of the total input impedance decreases and thereby the fre-

Figure 5. The impedances of CC and CE are large at low frequencies, and portions of signal voltages may be lost.

quency response characteristics are affected. The Miller effect is especially pronounced with common-emitter amplifiers, be­cause they introduce a 180° phase shift between the input and the output. For example, the values of CBE and CBC may be small, say 5 pF and 4 pF. But when the transistor is used in an amplifier with a gain of 99, the total input capacitance will be large enough to affect the frequency output characteristics of the amplifier. This is because

ci — cbe + (1 + av )(cbc)

= 5 + (1 + 99)(4) — 405 pF

It is recalled that at low frequencies the coupling capacitor and the emitter bypass capacitors offer high impedances and therefore portions of signal voltage may be lost, as shown in

Fig. 5.

A’ 7

rj V feedback

^O, Miller — A1 — 1

The Miller effect is thus an extremely important concept in discussing feedback. Equations for calculating the Miller input impedance and Miller output impedance can be devel­oped, and are given below: